The apparent absorbance of the incident radiation due to scattering. For small particles, direct proportionality exists between turbidity and the Rayleigh ratio.
Source:
Purple Book, 1st ed., p. 66 (http://old.iupac.org/publications/books/author/metanomski.html)
See also:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2218 (https://doi.org/10.1351/pac199062112167)